In U.S. Pat. Nos. 5,896,713 and 6,035,593, both of which are owned by the same assignee as is the present invention and both of which are incorporated herein by reference, tubular skylights are disclosed. Both of the skylights can use the skylight dome disclosed in U.S. Pat. No. 5,896,712 also owned by the same assignee as is the present invention and also incorporated herein by reference. These inventions represent advances over the prior art and one or more of them has found commercial success.
Briefly, a tubular skylight such as those mentioned above includes a tube assembly mounted between the roof and ceiling of a building. The top end of the tube assembly is covered by a roof-mounted dome or cover, such as the one disclosed in the above-mentioned '712 patent, while the bottom end of the tube assembly is covered by a ceiling-mounted diffuser plate. With this combination, natural light external to the building is directed through the tube assembly into the interior of the building to illuminate the interior.
Tubular skylights use a near specular finish reflective surface to transport sunlight down the tube from the roof to the interior ceiling. “Specular” means that reflected direct rays of sunlight maintain a near parallel beam of light as they reflect down the tube if the tube sides are parallel and the specular reflective surface is maintained.
The present invention understands that sunlight enters the tube at various incident angles based on time of day/year, latitude, and tube opening plane location. Despite the fixed position of the tube, direct beam sunlight reflects down the perimeter of the tube at approximately the same elevation angle as it enters the tube. As understood herein, this can result in the following undesirable outcomes. First, the parallel beam sunlight can converge at concentrated focal points at various locations down the tube, resulting in potentially dangerous hot spots that can cause fires particularly in the presence of combustible materials. Second, uneven illumination results at the base diffuser of the tube, because the perimeter path of the light rays in combination with the focal points can cause partial and non-uniform illumination of the diffuser. The result is poor illumination performance and glare from the diffuser. Additionally, the direct light beams passing through a prism in the diffuser can cause the separation of the wavelengths and project rainbows into the interior.
Accordingly, the present invention makes the following critical observations. Direct beam sunlight reflected from a specular surface in the approximate shape of a tube can create hot spots that are unsafe and that reduce product performance due to non-uniform illumination. As further understood herein, simply reducing the specularity of the tube results in reduced light transmission. Likewise, installing a diffuser above the tube to address glare and hot spots reduces the total system performance due to the transmission loss of this extra diffuser and the increased tube reflections caused by the light spread. Moreover, attempting to remedy the above-noted shortcomings using a random patterned reflector results in the light being diffused in a hemispherical shape that may send greater than 50% of the light back up the tube, therefore once again reducing performance. With the above observations in mind, the invention herein is provided.